The invention relates to a braking fluid pressure control device adapted to provide between front and rear wheels a braking force distribution approximating an ideal braking force distribution curve.
Generally, braking fluid pressure control devices are adapted to provide a braking force distribution between front and rear wheels such as to directly apply a braking fluid pressure to the front and rear wheels when the braking force applied to the front and rear wheels is below a predetermined level and to apply a reduced braking fluid force to the rear wheels when the braking force exceeds the predetermined level.
Braking fluid pressure control devices have already been proposed in the art which employ an inertia valve mechanism including an inertia ball and a valve seat on which the inertia ball is seated for detecting the point at which the control of the baking fluid pressure to be applied to the rear wheels is started. For example, one such control device cmploying an inertia ball is disclosed in U.S. Pat. No. 3,944,293 to Ishigami et al. In conventional fluid pressure control devices having such an inertia valve mechanism, however, it is difficult to provide an ideal braking force distribution between the front and rear wheels either during no load conditions or during load conditions since their proportioning valves are so constructed as to operate when the inertia ball is seated on the valve seat to block the braking fluid flow passage. This will be described in greater detail with reference to FIG. 1, wherein the reference letter a designates an ideal braking fluid pressure distribution curve under no load conditions and the letter b an ideal braking fluid pressure distribution curve under load conditions. It is desirable for the braking fluid pressure to be controlled in accordance with braking fluid pressure distribution curve A made up of line c and line d during no load conditions and in accordance with braking fluid pressure distribution curve B made up of line c and e during load conditions. In the conventional braking fluid pressure control device, however, if the inertia valve mechanism is so constructed that the inertia ball is seated on the valve seat at the reference point s of line c at which the braking deceleration becomes 0.2 g in order to control the braking fluid pressure in accordance with braking fluid pressure distribution curve A during no load conditions, the braking deceleration will become 0.2 g at the reference point t slightly higher than the reference point s during load conditions, although a braking fluid pressure larger than that indicated by the reference point s is required to provide a 0.2 g braking deceleration during load conditions, so that the braking fluid pressure is controlled in accordance with a braking force distribution curve made up of line c and line f during load conditions, which causes lack of braking force.
Furthermore, in the conventional braking fluid control device employing such an inertia valve mechanism, it is difficult to provide a sufficient braking force and the desired vehicle direction stabillity, particularly in a braking operation under no load condition. The reason for this is that although the inertia ball should be seated on the valve seat at a low vehicle deceleration where the braking fluid pressure distribution curve A is set in a very low position, the inertia ball will not be seated on the valve seat at times due to external factors, such as vehicle vibrations, the vehicle running on slopes or the like, so that the rear wheels will be supplied with an excessively large braking force resulting in skid or with an excessively small braking force resulting in lack of braking force.